Methods of this type in which a signal is gained from the processor which is proportional to the processed film or paper area and, thus, also proportional to the amount of silver deposited in the fixing bath of the processor, are known. In DE-PS 1 188 822, for example, a method is described for electrolytically recovering silver in which recovery is controlled by turning on and off the electrolytic current in response to the amount of photographic material moved through the fixing bath. The surface area of the material to be processed is the basic parameter therefor, assuming that on average always identical amounts of silver are dissolved from the material.
In DE-PS 1 237 789 the operating cycle of the silver recovery system is determined by a pulse generator activated by the film material moved through the processor, with the possibility of storing the pulses. In this case as well, the surface area of the material to be processed is the decisive factor and, as a result, only an average amount of silver to be recovered from the photosensitive material is used as a basic parameter.
The disadvantage of such methods is that the amount of silver in the fixing bath is averaged and used as a basic parameter by means of which the electrolytic silver recovery system is controlled.
On the other hand, silver recovery systems are known that operate in response to the silver content of the fixing solution forming an electronic reference value. The reference value may be, for example, the cathode potential of a reference electrode that is proportional to the silver concentration. There is a disadvantage, however, in that a continuity of the value determined by the silver content can hardly be realized over an extended period of time so that the recovery system is not very reliably and exactly controlled.